This invention relates to high temperature sprayable, non-metallic heat reflecting coatings and, more particularly, to a heat reflecting coating and coating mixture for reflecting heat energy in the frequency range of up to about 5 microns.
Good heat reflection is desirable from metal or ceramic type article substrates intended to experience high temperatures. One example of such an article is a component used in furnaces for melting glass or metals. Another example is an article used in certain power generating apparatus such as gas turbine engines which include components, made from a high temperature superalloy for example based on Ni or Co or both, operating at temperatures of up to 2000.degree. F. or more. One type of such a component includes the relatively large parts of the exhaust system in the hot operating aft section of an engine.
To protect such an article or component from the extreme high temperature environment experienced during engine operation, it had been a practice to include on the component outer surface an environmental resistant coating such as of a metal base or a ceramic base or their combination. One commonly used coating is a ceramic type thermal barrier coating (TBC). Generally the TBC is in a TBC system including a bond coat between the outer TBC and the underlying substrate. A commonly used type of TBC outer layer is a coating based on zirconia stabilized with yttria, for example about 93 wt. % zirconia stabilized with about 7 wt. % yttria. This general type of TBC has been reported in such U.S. Pat. No. 4,055,705--Stecura et al. (patented Oct. 25, 1977); U.S. Pat. No. 4,328,285--Siemers et al. (patented May 4, 1982); and U.S. Pat. No. 5,236,745--Gupta et al. (patented Aug. 17, 1993).
Although TBC coatings and systems work well as insulating layers for certain applications, alone they do not reflect from a surface significant energy in the high frequency band infrared (IR) range of about 2.6-5 microns. In addition, they are semi-transparent to such energy and have a relatively rough surface for certain applications in which a smooth surface is desired, for example in a fluid flow path or where a thin film type heat reflective overcoating is to be applied.
Many coatings have been reported for use in reflecting heat energy in the lower ultraviolet (UV) frequency range of up to about 2 microns. Some of such coatings, as they relate to the present invention, include coating pigments which include zinc compounds, such as the oxides or the titanates, in a mixture with a binder having silica, a silica compound, or a silicone which can form a silica compound, to resist degradation and/or chalking of the coating on exposure to such UV energy and/or elevated temperatures. For example such U.S. Pat. No. 3,337,358--Vossen (patented Aug. 22, 1967); U.S. Pat. No. 3,576,656--Webb et al. (patented Apr. 27, 1971); U.S. Pat. No. 3,607,338--Webb et al. (patented Sep. 21, 1971); U.S. Pat. No. 3,793,055--Shodai et al. (patented Feb. 19, 1974); and U.S. Pat. No. 4,039,347--Fletcher et al. (patented Aug. 2, 1977) each includes a binder including silica, a silica compound or a silicone for use in a coating, for example to reflect heat energy in the UV band range. However, it has been recognized in evaluations relating to the present invention that the presence in a coating, intended to reflect heat energy in the IR frequency range of about 2.6-5 microns, of silica or a silica compound or a material such as a silicone which will form silica or a silica compound upon exposure to elevated temperatures in air, will cut off heat reflectance at about 4 microns.